Measurement and analysis of vibration data is a well known method of monitoring the condition of machines. In an ideal machine, no vibration would be produced since all input energy would be used to perform useful work. In practice, however, vibration occurs as a normal by-product of the interaction of mechanical forces within the machine. A good machine design is one which produces low levels of inherent vibration. Subsequent increases in vibration level indicate a change in the dynamic characteristics of the machine, often caused by a defect or deterioration of moving parts.
Perhaps the earliest analyzer of vibration was the power plant attendant who made periodic inspections of plant equipment. This attendant, sometimes called a "runner" because he traveled throughout the entire plant, typically inspected vibration levels by placing his hand upon the machine or by simply listening for sounds produced by abnormal vibrations.
Modern technology has greatly simplified and improved upon vibration monitoring techniques. Sensitive accelerometers have replaced human hands in sensing vibration, and complex electronics have evolved to process the vibration data.
The prior art reveals several methods and apparatus for monitoring vibration. Some devices continuously monitor overall vibration in the time or frequency domain, and provide an indication of an alarm condition when preset vibration levels have been exceeded, (e.g., Shima et al., Judging System For Detecting Failure of Machine, U.S. Pat. No. 4,366,544, Dec. 28, 1982).
Another method of vibration analysis is "Vibration Signature Analysis", which is most often accomplished in the frequency domain. Under this method, time-domain vibration data are converted to the frequency domain using a Fourier Transform. The unique frequency spectrum obtained is often termed the "signature" of the machine. A signature of a machine under test may be analyzed and compared to a signature for a normal machine. Differences in the two spectra may indicate an abnormal condition. Prior art devices capable of providing a frequency spectra are known. One such device includes a handheld probe for collecting vibration data, and the capability of executing a Fast Fourier Transform to provide a frequency spectrum, (e.g., Microlog IMS, available from Palomar Technology International, Carlsbad, Calif.). Morrow also discloses a data acquisition system which performs an automatic frequency spectrum analysis whenever a probable or actual malfunction is detected (Morrow, Data Acquisition System, U.S. Pat. No. 4,184,205, Jan. 15, 1980).
A common problem associated with most of the prior art monitoring equipment is that they usually require a human operator to analyze and compare the signatures. Prior art inventions lack the sophisticated electronic circuitry and data processing necessary for automatic comparison of the spectra and for rendering a decision regarding the condition of the machine under test, with only minimal human interface. Prior art inventions are also generally incapable of analyzing machines under transient conditions, and thus find applications restricted to steady state operation. Still other prior art devices are incapable of extracting events, or specific sections of interest in a typical vibration signal. Also, many prior art devices are large and bulky, or require interfacing with a mainframe computer. Finally, most prior art devices require the sensing element (e.g., accelerometer) to be located proximate the machine element to be analyzed. For example, bearings on electric motors are typically monitored by placing sensing devices on the bearing housings themselves.